The Motion of Trapped Holes on Nanocrystal Surfaces

Abstract

This Perspective discusses the phenomenon of trapped-hole diffusion in colloidal semiconductor nanocrystals. Surface charge-carrier traps are ubiquitous in nanocrystals and often dictate the fate of photoexcited carriers. New measurements and calculations are unveiling the nature of the nanocrystal surface, but many challenges to understanding the dynamics of trapped carriers remain. In contrast to the view that trapped holes are stationary, we have put forward a series of reports demonstrating that trapped holes on the surfaces of CdS and CdSe nanocrystals are mobile and move between traps in a sequence of hops. We summarize how these findings advance the understanding of carrier dynamics in colloidal nanocrystals and how they may impact a broad set of excited-state behaviors in these materials.

Ternary Dumbbell Nanowires for Photocatalytic Hydrogen Production

“…Nonetheless, the aforementioned photoelectrochemical measurements confirm that electrons accumulate successfully at the surface of the IrSe 2 @MoSe 2 core-shelled tips, where they promote the HER reaction, indicating that internal recombination with holes within the IrSe 2 is not the dominant process. Possibly, this is the result of a slow hole migration that enables successful extraction of the holes by the sacrificial hole scavengers before such interfacial transfer may occur . A control in which lactic acid is removed was performed, confirming that no H 2 generation is observed in the absence of the hole scavenger.…”

“…Possibly, this is the result of a slow hole migration that enables successful extraction of the holes by the sacrificial hole scavengers before such interfacial transfer may occur. 79 A control in which lactic acid is removed was performed, confirming that no H 2 generation is observed in the absence of the hole scavenger. To further confirm the proposed electron transfer pathway, the catalytic activity of the TDNWs was evaluated, utilizing a 530 nm LED as a light source to exclusively excite the IrSe 2 @MoSe 2 tips.…”

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

“…For example, recombination between a trapped hole and bulb-localized electron in a CdS NNR, CdSe NNR, ZnSe/CdS DiR NNR, or CdSe/CdS DiR NNR is not complete for tens of microseconds (Figure 36d, blue trace). 97,190,214,279,333 As noted in Section 5.4.1, surface-trapped holes in CdS and CdSe NRs and heterostructure NRs are not stationary at one trap site but instead diffuse along the NR surface with hops on the order of individual bond lengths. Hence, in CdS, CdSe, CdSe/CdS, and ZnSe/CdS NNRs, the recombination of bulblocalized electrons with holes trapped to the rods (Figure 36a) is diffusion-limited.…”

“…The charge-separated state in the CdS NNR greatly extends the excited state lifetime. Reprinted with permission from ref . Copyright 2020 American Chemical Society.…”

Carbonate-Ion-Mediated Photogenerated Hole Transfer to Boost Hydrogen Production

“…Water photocatalysis to clean fuels under sunlight provides a potential alternative to mitigate the ever-rising global energy crisis and related environmental problems. , A two-electron half-reaction for water reduction (2H + + 2e – → H 2 ) has been carefully explored to leverage photogenerated electrons created from a semiconductor catalyst to achieve hydrogen gas. , Kinetically, in water photocatalysis, the rapid recombination of photogenerated holes and electrons constrains the hydrogen generation rate. − Despite impressive efforts being made to transfer or consume the holes by introducing sacrificial agents serving as a hole scavenger, the hole-transfer kinetics remains a major obstacle for steady and continuous hydrogen generation. , Most promising implementation for hydrogen photosynthesis focuses on metal sulfides, , oxysulphides, or phosphides, ,− even though the oxidation of their surface by the photogenerated holes can be still observed under light illumination, ,, which severely limits the stability of these photocatalysts.…”